This invention relates to a thermoelectric element which is mainly composed of an iron silicide and which can be used for power generation utilizing various kinds of heat sources, and a method of manufacturing same, and more particularly to a thermoelectric element and a method of manufacturing same, which can be used in a power source for holding open an electromagnetic valve for gas appliances such as a gas heater or the like.
In general, a thermoelectric element of this kind has a U-shaped structure as shown in FIG. 1, which comprises a pair of parts 1 and 2 formed, respectively, of a p-type iron silicide containing Mn, and an n-type iron silicide containing Co and joined together at ends thereof to form a pn junction 3 there, with a gap 4 defined between the parts 1, 2.
However, the thermoelectric element having such a U-shaped structure has insufficient mechanical strength at the pn junction 3 such that it can sometimes be broken even when a small external force is applied thereto, making it greatly inconvenient to handle the element in assembling and using same.
To eliminate such inconvenience, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 56-152282 to interpose an insulating layer formed of an oxide such as forsterite (2MgO.SiO.sub.2), aluminum oxide, magnesium oxide, and zirconium oxide between a layer of the p-type iron silicide and a layer of the n-type iron silicide along part of the entire length of the layers to form a laminated body with a pn junction at one end thereof, and sinter the laminated body into a thermoelectric element having a p-type iron silicide layer 1, an n-type iron silicide layer 2, and an intermediate insulating layer 5 as shown in FIG. 2. In the thermoelectric element thus obtained, the p-type iron silicide layer 1 and the n-type iron silicide layer 2 are joined together via the intermediate insulating layer 5, and therefore the pn junction 3 has considerably higher mechanical strength as compared with the thermoelectric element of the U-shaped structure shown in FIG. 1.
Among the above-mentioned insulating oxides which can be interposed between p-type and n-type iron silicide layers at portions thereof other than a pn junction-forming portion thereof, the forsterite (2MgO.SiO.sub.2) has a coefficient of thermal expansion of 10-12.times.10.sup.-6 /.degree. C., which is close to that of 10.times.10.sup.-6 of an iron silicide (FeSi.sub.2), and is therefore suitable for use as the insulating layer of thermoelectric element. However, the forsterite (2MgO.SiO.sub.2) cannot be sintered at a temperature not lower than 1300.degree. C., whereas the melting point of the iron silicide (FeSi.sub.2) is 1220.degree. C. Consequently, if the laminated body formed of three layers of p-type iron silicide, n-type iron silicide, and forsterite is sintered at a temperature of 900.degree.-1200.degree. C. which is a suitable sintering temperature for the iron silicide (FeSi.sub.2), the insulating layer formed of forsterite is not sintered to a sufficient degree, and hence the sintered insulating layer will have low density as well as insufficient strength. Consequently, the thermoelectric element having such an insulating layer of forsterite with low density has low thermal shock resistance. Further, the thermoelectric element has insufficient mechanical strength and can be broken during operation of installing a gas appliance, or oil can permeate the insulating layer during use of the gas appliance to degrade the degree of insulation.